Works in reverse. When in forward it does not engage or only weakly. Then get revs up to about 1200 and it slams into gear. The cables are fine and the lever on the side of gear box moves easily and to the fullest extent.

Sounds like truly an internal problem. The oil (SAE 30 HD) is a bit black but doesn't smell burnt.
A friend suggests flushing numerous times with say SAE 10 to see if anything is stuck or "plugged". I've no idea what's inside!!
Any ideas?
thanks, John